I know that the world horse population peaked just before WW1 and that horses died at a staggering pace all over the continent during the war. Dying from poor conditions, overwork, lack of food, and even being blown up by artillery.
However, I also know that the industrialized world was moving towards more reliance on the automobile/tractor than the horse during the same period. Also that horses were being phased out of military use in the UK at this time.
There's multiple kinds of horses, so one suited to pulling a plow or a carriage will suffice.
Ok! So. Here's what I found:
1914 Type of horse|Price (In pound sterling)
Officer's mount | 70
Trooper's mount | 50
Mounted infantry | 50
Heavy draught (pulling horse) | 60
This isn't exactly the horse's actual value. These numbers show how much the British military was willing to pay for a horse in 1914.
In about 1938, a draught horse would have cost about 65 pounds stirling.